This invention relates to spark timing systems for spark ignited, internal combustion engines and particularly for those which have apparatus for generating conventional spark timing pulses from engine crank angle, speed and manifold vacuum inputs and retarded spark pulses corresponding to said conventional spark timing pulses but retarded therefrom by a crank angle determined by one or more other engine operating conditions.
One method of providing the retarded spark timing pulses, shown in the prior art, is the technique of varying the voltage across a capacitor in a first direction toward a reference voltage at a first rate, generating a retarded spark pulse when a voltage reaches the reference voltage and, when the retarded spark pulse is generated, reversing the direction of voltage change and changing the voltage in the opposite direction at a second rate until the next conventional spark timing pulse. In this techinque, one of the first and second rates is constant while the other may be constant or vary according to one or more sensed engine operating conditions; and the amount of retard for each retarded spark timing pulse is thus controlled as a function of crank angle independently of engine speed.
It may be found desirable, with such a system to generate the retarded spark timing pulses during engine start as well as normal running conditions. Engine speed during at least the beginning of its starting mode, however, is determined by the speed of the cranking motor and is normally significantly slower than normal engine idle speed. A typical engine cranking speed might be 200 rpm, compared with a sample normal engine operating speed range of 700 rpm at idle to 5,000 rpm maximum. In the system described above, the voltage across a capacitor changes in the opposite direction from the reference to a peak voltage before it is change back toward the reference. This peak voltage varies with engine speed, becoming greater as the engine speed decreases. Since the normal range of engine speeds is already great, the system is normally designed so that the maximum difference between the peak voltage and the reference, which occurs at idle speed, normally about 700 rpm, is close to the maximum voltage that the vehicle electrical system can produce across the capacitor. Thus, operation at cranking speed of 200 rpm would require the system to charge the capacitor to almost three times the maximum available DC voltage. This is, of course, impossible; and the voltage across the capacitor would stop changing when the maximum was reached, thereby clipping and distorting the retard angle of the retarded spark timing pulses.